The Chandley U.S. Pat. No. 4,112,997 issued Sept. 12, 1978, illustrates the countergravity casting of molten metal in a gas permeable shell mold wherein the lower end of a riser passage is submerged in a molten metal pool, a reduced pressure is applied to a plurality of mold cavities through the gas permeable walls of the mold to urge molten metal to flow upwardly through a stabilizing and filtering screen in each ingate to each mold cavity to fill each mold cavity with molten metal. After the mold cavities are filled with molten metal and most of the casting has solidified, the mold is removed from the molten metal pool with the reduced pressure maintained on the mold cavities. Upon removal of the mold from the molten metal pool, the molten metal in the riser passage and in the portion of the ingates between the stabilizing and filtering screen and the riser passage drains from the mold by gravity-induced run-out before the molten metal in the mold cavities is completely solidified. The molten metal in the mold cavities and in the portion of the ingates between the stabilizing and filtering screen and the mold cavity is held against run-out by the reduced pressure applied on the mold cavities and by the stabilizing effect of the stabilizing and filtering screens on the molten metal. After at least a solidified skin of metal is formed in the mold cavity and in the portion of the ingates between the screen and the mold cavity, the reduced pressure applied to the mold is released. However, as a result of the small dimension of the stabilizing and filtering screen in the direction of molten metal flow, the reduced pressure must be applied to the mold cavities for a relatively long time, e.g., 200 seconds, until the solidified skin forms in the mold cavity and in the portion of the ingates between the screen and the mold cavity. This prolongs the casting cycle time, and reduces the rate of production of solidified castings. Moreover, stabilizing and filtering screens suitable for use in the casting of high melting point metals (e.g., metals having melting temperatures above about 2950 F.) are expensive and increase the cost of the castings so produced.
The Chandley et al U.S. Pat. No. 4,589,466 issued May 20, 1986, illustrates the countergravity casting of molten metal wherein a gas permeable mold includes a crimpable fill pipe sealingly connected to the lower end of the riser passage and adapted for immersion in an underlying molten metal pool during casting to fill a plurality of mold cavities in the mold. Once the mold cavities are filled with molten metal by countergravity casting from the underlying casting melt, the fill pipe is crimped closed while immersed in the molten metal pool to prevent molten metal run-out upon subsequent removal of the fill pipe from the molten metal pool. Molten metal remains and solidifies in the fill pipe above the crimped portion and in the mold cavities, the intermediate riser passage and the ingates to each mold cavity. In the casting of higher melting point metals the use of a crimpable fill pipe provides an unsatisfactory degree of reliability since the hot metal can occasionally melt through the fill pipe even when it is coated with a ceramic wash or layer. Moreover, the crimped fill pipe is not reuseable.
The Sylvester U.S. Pat. No. 3,032,841 issued May 8, 1982, illustrates in one embodiment an ingate structure through which molten metal is supplied in countergravity fashion to fill a plurality of gas impermeable molds. A stopper valve is disposed in the ingate structure between a depending fill tube and the mold cavities and is movable in the ingate structure to a closed position after the mold cavities are filled to prevent molten metal run-out. After the stopper is moved to the closed portion, the molten metal in the ingate passages above the valve is allowed to at least partially solidify to substantially close the ingate passages. Thereafter, the molds and the ingate structure are separated as a unit from the fill tube and then the molds are subsequently separated from the ingate structure. The patent indicates that the viscosity and surface tension of molten metal, if any, in the restricted (partially closed) ingate passages prevents run-out of molten metal therefrom, even though the metal above and below the ingate passages may still be in the molten state.
It is an object of the invention to provide a method and apparatus for the differential pressure, countergravity casting of molten metal in substantially shortened cycle times by differential pressure, countergravity filling a mold having a mold cavity and a constricted molten metal inlet means for supplying the molten metal to the mold cavity when a lower mold portion is immersed in an underlying molten metal pool and then withdrawing the mold from the pool while holding the molten metal in the inlet passage means which is so constricted in size as to coact with a differential pressure maintained on the molten metal in the mold to substantially prevent molten metal run-out from the mold before the metal solidifies in the inlet passage means or before the mold is inverted.
It is another object of the invention to provide a method and apparatus for the differential pressure, countergravity casting of molten metal in substantially shortened cycle times by differential pressure, countergravity filling of a mold having a bottom fill passage immersed in an underlying molten metal pool, withdrawing the mold from the pool and draining the molten metal from the fill passage while the molten metal in the mold remains liquid and unsolidified and is held in constricted inlet passage means in the mold above the fill passage by a combination of differential pressure and molten metal surface tension holding action applied to the molten metal in the constricted inlet passage means.
It is another object of the invention to provide a method and apparatus for the differential pressure, countergravity casting of molten metal using a bottom fill tube on the mold that can be drained of molten metal following filling of the mold cavities to lessen the amount of metal used in the gating of the casting and that is removable after the mold cavity is filled with molten metal for reuse in the casting of successive molds.